Evening out the spin and charge parity to increase $${T}_{c}$$ in $${{\rm{Sr}}}_{2}{{\rm{RuO}}}_{4}$$

Abstract

Unconventional superconductivity in Sr 2 RuO 4 has been intensively studied for decades. However, the nature of pairing continues to be widely debated. Here we develop a detailed ab initio theory, coupling quasiparticle self-consistent GW approximation with dynamical mean field theory (DMFT), including both local and non-local correlations to address the subtle interplay among spin, charge and orbital degrees of freedom. We report that the superconducting instability has multiple triplet and singlet components. In the unstrained case the triplet eigenvalues are larger than the singlets. Under uniaxial strain, the triplet eigenvalues drop and the singlet components increase. This is concomitant with our observation of spin and charge fluctuations shifting closer to wave-vectors favoring singlet pairing. We identify a complex mechanism where charge fluctuations and spin fluctuations cooperate in the evenparity channel under strain leading to increment in critical temperature (Tc), thus proposing a novel mechanism for pushing the frontier of critical temperature (Tc) in unconventional 'triplet' superconductors.

Controlling Tc through Band Structure and Correlation Engineering in Collapsed and Uncollapsed Phases of Iron Arsenides

“…We conclude that UT-CFA would be superconducting if it did not make a transition to an antiferromagnetically ordered state. The superior quality of the QSGW bath combined with nonperturbative DMFT has been shown to possess a high degree of predictive power in one-and twoparticle spectral functions [7][8][9]12] and as in other cases we are able to replicate the experimental observations of spectral functions, including a reasonable estimate for T c . The remainder of the Letter uses this machinery to explain what the origins of superconductivity are.…”

“…Superconductivity depends critically on the alignment of this state to the Fermi level. We are able to make these findings thanks to recent developments that couple (quasiparticle) self consistent GW (QSGW) with dynamical mean field theory (DMFT) [7][8][9][10]. Merging these two state-of-the-art methods captures the effect of both strong local dynamic spin fluctuations (captured well in DMFT), and nonlocal dynamic correlation [11,12] effects captured by QSGW [13].…”

“…On top of the DMFT self-energy, charge and spin susceptibilities are obtained from vertex functions computed from the two-particle Green's function generated in DMFT, via the solutions of the nonlocal Bethe Salpeter equation. Additionally, we compute the particle-particle vertex functions and solve the linearized Eliashberg equation [9,14,15] to compute the superconducting susceptibilities and eigenvalues of superconducting gap instabilities. For CT and UT phases we use a single value for U and J (3.5 and 0.62 eV, respectively), which we obtained from bulk FeSe (and LiFeAs) within a constrained RPA implementation following Ersoy et al [16].…”

“…Expressions we use for the response functions are presented in Ref. [9]. Our all-electronic GW implementation was adapted from the original ecalj package [18]; the method and basis set are described in detail in PHYSICAL REVIEW LETTERS 124, 237001 (2020) 0031-9007=20=124( 23)=237001( 5)…”

Momentum-resolved superconducting energy gaps of Sr 2 RuO 4 from quasiparticle interference imaging

“…However, although critical to testing advanced theories [18][19][20][21][22][23] for superconductivity in Sr2RuO4, the k-space structure of Δ ( ); Δ ( ); Δ ( ) has never been measured directly. Basically, this is because the maximum magnitude of any of these gaps 30,31 is |Δ| ≤ 350 so that temperature ≲ 100 and energy resolution with ≲ 100 are required to spectroscopically detect strongly anisotropic k-space gap structures and/or their gap minima.…”